A method for labeling sensitive biological molecules such as proteins with tritium is proposed. The method consists of bombarding a sample, which has been lypholized or evaporated on a stainless steel plate, in a vacuum chamber with vibrationally excited tritium species having high translational energies. Reactions of these excited species with the solid sample to be tritiated occurs in a random fashion with both exchangeable and non-exchangeable protons. After tritiation the exchangeable tritium atoms are removed from the sample by treatment with water. The reaction can be applied to any organic molecule which has accessible C-H bonds. The tritiation process is rapid and does not take the extreme times typical of the Wilzbach method nor does the process lead to decomposition of appreciable amounts of sample as in the discharge methods. The method is being applied to the preparation of tritiated proteins. Thermolysin, collagen, alpha 1-antiprotease inhibitor and antithrombin II are some of the proteins which have been tritiated.